Selective catalytic reduction of N2O

ABSTRACT

A gas stream containing nitrous oxide and ammonia is contacted with a catalyst composition containing a zeolite. N 2 O is reduced to N 2  and H 2 O at low temperatures in a highly efficient manner. Ammonia-mediated reduction of nitrous oxide can be effectuated from gas streams having N 2 O concentrations as low as 1%. The gas stream may also contact a catalytic composition selective for the reduction of NO x . In this way, N 2 O and N x  treatment may be effectuated in a single process stream.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to an improved method for the reduction of nitrous oxide; and more particularly, to ammonia-mediated reduction of nitrous oxide.

2. Description of the Prior Art

Nitrous oxide (N₂O) is not commonly considered an atmospheric pollutant and has not been considered a constituent of the gaseous pollutants collectively referred to as nitrogen oxides (NO_(x)) which have received wide attention as pollutants harmful to the environment. However, recent studies indicate that N₂O in the Earth's atmosphere may be increasing at a rate of about 0.2% per year and that this increase appears to be caused by anthropogenic activity.

N₂O is a major stratospheric source of NO, is believed to be involved in destroying the ozone layer and is recognized to be a greenhouse gas. Because N₂O has an atmospheric lifetime of approximately 150 years, researchers are attempting to identify sources of the pollutant and to limit further production of the harmful gas. Recent reports such as an article by Thiemens and Trogler, Science, 251(1991)932 suggest that various industrial processes significantly contribute to the increased levels of N₂O found in the Earth's atmosphere.

For example, nitrous oxide is a by-product formed during the manufacture of monomers used in producing 6,6- and 6,12-nylon. Nylon polymers are typically formed by subjecting a dicarboxylic acid and a diamine to a condensation polymerization reaction. The most widely used dicarboxylic acid, adipic acid, is prepared primarily by oxidizing cyclohexane in air to form a cyclohexanol/cyclohexanone mixture followed by oxidizing such mixture with HNO₃ to form adipic acid and N₂O. Thiemens and Trogler calculate that about 1 mol of N₂O per mole of adipic acid is formed as a side product in adipic acid processes. Assuming that 2.2×10⁹ kg of adipic acid are produced globally per year, about 1.5×10¹⁰ mol yr⁻¹ of N₂O by-product or 10% of the annual output of atmospheric N₂O can be attributed to this single process. Also, for many industrial processes, N₂O may be co-present with nitrogen oxides, NO_(x) (NO and NO₂), in the effluent gases.

M. Schiavello and coworkers, (J. Chem. Soc. Faraday Trans. 1, 71(8), 1642-8) studied various magnesium oxide-iron oxides and magnesium oxide-iron oxide-lithium oxide systems as N₂O decomposition catalysts. While magnesium oxide-iron oxide samples which were fired in air and which contained MgFe₂O₄ demonstrated low activity, similar samples fired under reducing atmospheres and containing Fe²⁺ in solid solution demonstrated greater activity. The researchers concluded that Fe³⁺ ions in the ferrite phase are not catalytically active toward the subject reaction whereas Fe³⁺ ions contained in MgO together with Li⁺ are catalytically active when the ratio of lithium to iron is less than 1.

P. Porta and coworkers (J. Chem. Soc. Faraday Trans 1, 74(7), 1595-603) studied the structure and catalytic activity of Co_(x)Mg_(1-x)Al₂O₄ spinel solid solutions for use as catalysts in decomposing N₂O into gaseous nitrogen and oxygen. The catalytic activity per cobalt ion in various N₂O decomposition catalysts was found to increase with increasing dilution in MgO. The distribution of cobalt ions among octahedral and tetrahedral sites in the spinel structure of Co_(x)Mg_(1-x)Al₂O₄ was found to vary with temperature and the fraction of cobalt ions in octahedral sites was found to increase with increasing quenching temperature. The researchers concluded that catalytic activity generally increases as a greater amount of cobalt ions is incorporated into octahedral sites in the structure.

W. Reichle (Journal of Catalysis 94 (1985) 547) reported that various anionic clay minerals belonging to the pyroaurite-sjogrenite group, such as hydrotalcite (Mg₆Al₂(OH)₁₆(CO₃ ²⁻)·4H₂O can be thermally decomposed to form a product which is a useful catalyst for vapor-phase aldol condensations. Replacement of Mg by Fe, Co, Ni and Zn and/or replacement of Al by Fe and Cr also results in isomorphous double hydroxides which, on heat treatment, are rendered catalytically active. The reference also states that the activity of the catalyst is strongly affected by the temperature at which the hydrotalcite is activated.

Commonly owned U.S. Pat. No. 5,171,553 discloses a highly efficient, commercially viable process for removing N₂O from gaseous mixtures. The process utilizes catalysts comprising a crystalline zeolite which, at least in part, comprise five membered rings having a structure type selected from the group consisting of BETA, MOR, MFI, MEL and FER wherein the crystalline zeolite has been at least partially ion-exchanged with a metal selected from the group consisting of copper, cobalt, rhodium, iridium, ruthenium and palladium.

Likewise, commonly owned U.S. Pat. No. 5,407,652 discloses an efficient catalytic pollution control process for removing N₂O from gaseous mixtures. The process utilizes catalysts derived from anionic clay minerals such as hydrocalcites, sjogrenites and pyroaurites which, after appropriate heat activation, provide superior N₂O decomposition activity.

While the prior art has shown an awareness of the decomposition of N₂O into its respective components, industry urgently needs to develop enhanced catalytic processes for destroying N₂O emissions prior to the venting of commercial process effluent streams into the atmosphere. This need is particularly critical with respect to effluent streams containing low levels of this contaminant. In addition, methods are needed to remove this contaminant from engine exhaust streams. It would be particularly useful if the catalytic decomposition of N₂O could be combined with reduction of NO_(x) so as to economically and efficiently remove these pollutants from both industrial effluent streams and engine exhaust streams.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method for ammonia-mediated reduction of nitrous oxide comprising contacting a gas stream containing nitrous oxide and ammonia with a catalyst composition comprising a zeolite. Advantageously, N₂O is reduced to N₂ and H₂O at lower temperatures and with greater efficiency than heretofore known in the art. In this way, the present invention provides an economical and reliable control method for nitrous oxide pollution.

In another aspect of the invention there is provided a method for ammonia-mediated N₂O and NO_(x) reduction comprising contacting a gas stream containing ammonia with a catalyst composition containing an upstream catalyst and a downstream catalyst as sensed relative to the sequence of flow of the gaseous stream through the catalyst wherein the upstream catalyst is selective for the reduction of NO_(x) and the downstream catalyst is selective for the reduction of N₂O. Alternatively, this catalyst configuration may be reversed. Advantageously, the upstream and downstream catalysts can comprise the same material. The ability to control N₂O and NO_(x) in a single process stream and, where desired, with a single catalytic material, results in significant cost savings. Such a combined process is particularly useful in industries and in engine exhaust streams where N₂O and NO_(x) are present in the outgas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description and the accompanying drawings, in which:

FIG. 1 is a graph of laboratory data showing % N₂O conversion versus catalyst temperature for a zeolite based Fe-exchanged catalyst;

FIG. 2 is a graph depicting % ammonia conversion versus catalyst temperature for the same test series as in FIG. 1;

FIG. 3 is a graph depicting % N₂O conversion versus catalyst temperature for a V₂O₅/TiO₂ catalyst;

FIG. 4 is a graph depicting % ammonia conversion versus catalyst temperature for a V₂O₅/TiO₂ catalyst;

FIG. 5 is a graph depicting % N₂O conversion versus catalyst temperature for a Pt/Au catalyst;

FIG. 6 is a graph depicting % ammonia conversion versus catalyst temperature for a Pt/Au catalyst;

FIG. 7 is a graph of laboratory results showing the use of a zeolite catalyst to remove NO_(x) and N₂O in the absence of NH3.

FIG. 8 is a graph of laboratory results showing the use of a zeolite catalyst to achieve reduction for both NO_(x) and N₂O gases by introducing NH₃ into the gas stream;

FIG. 9 is a schematic of the apparatus for the control of N₂O gas; and

FIG. 10 is a schematic of the apparatus for the control of N₂O and NO_(x).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a highly efficient catalytic method for converting nitrous oxide (N₂O) into environmentally safe products, namely gaseous nitrogen and water. The method is based on the surprising discovery that adding ammonia to a gas stream containing N₂O and passing the mixture over a zeolite catalyst composition results in enhanced N₂O reduction than otherwise known in the art. Specifically, the N₂O decomposition rate is enhanced when ammonia is introduced to the gas stream. In so doing, the introduced ammonia is also converted into N₂ and water, with its own conversion rate increased by the presence of N₂O. As a result of this enhanced N₂O decomposition rate, the method is capable of reducing N₂O at much lower temperatures and/or lower catalyst volumes than presently required. Moreover, the enhanced N₂O removal offered by the present invention permits the removal of low levels of N₂O (i.e., less than 1% N₂O) from process streams. The method can also be combined with selective catalytic reduction (SCR) of nitrous oxides (NO_(x)) to achieve simultaneous removal of N₂O and NO_(x). This is particularly advantageous for internal combustion engines or industrial processes where N₂O and NO_(x) are both present in the outgas.

In accordance with the present invention, there is provided a method for the reduction of a gas stream containing nitrous oxide which comprises contacting a gas stream containing N₂O with ammonia over a catalyst composition comprising a zeolite in order to catalyze the reduction of the N₂O with ammonia. Such gaseous streams, for example, the products of combustion of internal combustion engines, boilers, and the nitric acid manufacturing process often inherently contain substantial amounts of oxygen. These exhaust gases contain from about 2 to 15 volume percent oxygen and from about 20 to 100,000 volume parts per million (ppm) N₂O. Zeolites, and in particular, metal-promoted zeolites can be used to promote the reaction of ammonia with N₂O to form nitrogen and H₂O selectively over a competing reaction of oxygen and ammonia.

It is desirable in the method to provide sufficient ammonia to react completely with the N₂O present in order to drive the reaction to completion. However, in practice, significant excess ammonia is normally not provided because the discharge of unreacted ammonia from the catalyst to the atmosphere would itself engender an air pollution problem. Accordingly, the ratio of ammonia to N₂O in the gas stream should range up to about 2.0 ppm NH₃/ppm N₂O based on the total volume of the gas stream in order to impact N₂O reduction. The NH₃/N₂O ratio should be at least 0.5, with the most preferred ratio being from about 0.8 to about 1.0 NH₃/N₂O. Addition of the foregoing amounts of ammonia to the gas stream advantageously enhances N₂O conversion over process streams lacking ammonia.

As previously indicated, the method of the present invention is useful for ammonia-mediated N₂O reduction at lower processing temperatures than previously known in the art. In accordance with the present invention, N₂O reduction can be enhanced over conventional processes at temperatures of greater than 250° C. and preferably those ranging from 350° C. to 600° C. and most preferably 450° C. to 600° C.

Generally, any suitable zeolitic material may be utilized in the catalyst compositions of the invention. Preferably, the zeolitic materials are ion-exchanged with Fe, Cu, Co, Ce, Pt, Rh, Pd, Ir, Mg. Ion-exchanging the zeolitic materials serves to enhance the catalytic activity toward and selectivity for the reaction between NH₃ and N₂O. Useful zeolites for practice of the present invention include crystalline zeolites which, at least in part, comprise five membered rings having a structure type selected from the group consisting of BETA, MOR, MFI, ZSM, MEL, FER and Y. Of these, BETA, ZSM, MOR and Y are particularly preferred, and BETA is the most preferred. Suitable ion exchange compounds include Fe, Cu, Co, Ce, Pt, Rh, Pd, Ir, and Mg, with Fe, Cu, Co, Ce, Pd, Rh and Fe, Ce, Cu, Co and combinations thereof most preferred. Such ion-exchange techniques are well known in the art and are reviewed in Breck, Zeolite Molecular Sieves, Structure, Chemistry and Use, Chapter 7, Ion-Exchange Reactions and Zeolites, beginning on page 529, published by John Wiley and Sons, New York, 1974 which is expressly incorporated herein by reference.

Because the invention is capable of reducing N₂O at lower temperatures, removal efficiencies of >90% can be attained at temperatures of 300° C. to 600° C., with removal efficiency calculated as [[(inlet N₂O (moles)—outlet N₂O (moles))/inlet N₂O moles]* 100]. In addition, the method of the present invention can also remove small quantities of N₂O from gas streams having low levels of this contaminant; for example, less than 1%. Advantageously, the method of the present invention can remove N₂O from gas streams containing as much as about 5000 ppm N₂O to as low as 20 ppm at these temperatures.

The method described herein may also be combined with selective catalytic reduction of NO_(x) to achieve simultaneous removal of NO_(x) and N₂O from a single process stream. For example, many industrial process streams and engine exhausts contain NO_(x) and N₂O. Prior to the present invention, there was no suitable method for controlling both of these contaminants in a single process stream, due to the fact that most SCR catalysts are operated at less than 550° C. and no suitable catalyst was found to destroy N₂O effectively at those temperatures.

Where it is desired to simultaneously remove NO_(x) and N₂O from a single process stream, ammonia is introduced into the process stream upstream of the catalyst bed. The catalyst bed contains an upstream catalyst and a downstream catalyst. Three arrangements can be made to achieve the simultaneous removal. In one arrangement, the upstream and downstream catalysts comprise the same material which can promote both the selective reduction reaction of NO_(x) and reduction of N₂O. For this arrangement, a zeolite-based SCR catalyst, such as Fe/Beta, can be very effective. The ability to control NO_(x) and N₂O in the same reactor will result in significant savings in control costs. For this arrangement, the exhaust temperature is preferably in the range of 350° C. to 600° C. The second arrangement comprises an upstream catalyst which is selective for the reduction of N₂O and a downstream catalyst selective for the reduction of NO_(x). In this arrangement, the ammonia required for N₂O and NO_(x) removal is introduced before the upstream N₂O catalyst, which gives higher NH₃ concentration to further enhance the N₂O removal rate. The unconverted ammonia is then reacted with NO_(x) over the downstream SCR catalyst and converted to N₂ and water. This arrangement is advantageous for streams that contain a high concentration of N₂O relative to NO_(x), since the downstream SCR bed is used to convert the low level of NO_(x) and NH₃. The third arrangement comprises an upstream catalyst which is selective for the reduction of NO_(x) (SCR NO_(x)) and a downstream catalyst which is selective for reduction of N₂O. This arrangement is advantageous for treating streams that contain high concentrations of NO_(x) relative to N₂O. The unconverted ammonia coming off of the catalyst selective for the reduction of NO_(x) is used to promote reduction of N₂O over the downstream catalyst. Methods and suitable catalytic materials for removing NO_(x) are well known in the art and are described in detail in commonly owned U.S. Pat. No. 5,024,981 of Barry K. Speronello et at., entitled “Staged Metal-Promoted Zeolite Catalysts and Method for Catalytic Reduction of Nitrogen Oxides Using Same,” and commonly owned U.S. Pat. No. 4,961,917 of John W. Byrne entitled “Zeolite Catalysts and Method for Reduction of Nitrogen Oxides With Ammonia Using Same,” both of which are expressly incorporated herein by reference.

In this aspect of the invention, the catalysts are arranged in at least two zones in which one zone contains a catalyst selective for the reduction of NO_(x), and the other zone contains a zeolite catalyst selective for the reduction of N₂O. Any suitable form of the catalyst may be used in these and other aspects of the invention, such as a monolithic honeycomb-type body containing a plurality of fine parallel gas flow passages extending therethrough, the walls of which are coated with the catalytic material. Typically, such monolithic bodies are made of a refractory ceramic material such as cordierite, mullite or aluminia, and the catalytic material coating the fine gas flow passages is contacted by the gaseous stream as it flows through the gas flow passages. Separate monolith bodies may be used for each of the zones. As indicated above, each of the zones preferably comprises the same catalytic material.

The catalyst may also take the form of a packed bed of pellets, tablets, extrudates or other particles of shaped pieces, such as plates, saddles, tubes or the like. The physical configuration of the catalyst used in a given case will depend on a number of factors such as the space available for the catalytic reactor, the activity of the catalytic material utilized, and the permitted or desired amount of pressure drop across the catalyst bed; for example, where the method is used to treat engine exhausts. A preferred physical configuration of the catalyst is one which provides parallel flow passageways for the gas, such as those found in the above-described honeycomb-type catalysts. Other arrangements providing such parallel flow passageways include the use of parallel plates or stacked tubes. Because of its ease of handling and installation as well as good mass transfer characteristics relative to other parallel passage configurations, a highly preferred physical configuration of the catalysts of the invention is a monolithic honeycomb member having relatively high cell (flow passageway) density of approximately 60 cells or more per square inch of end face of the honeycomb member. The walls defining the gas flow passages (or cells) are desirably as thin as possible consistent with the requisite mechanical strength of the honeycomb. Catalysts used in the invention may take the form of a monolithic honeycomb carrier, the gas flow passages of which comprise or are coated with separate zeolite catalytic compositions as described above. For example, a catalytically inert honeycomb member, such as a cordierite carrier, may be coated with a washcoat of fine particles of a catalyst selective for the reduction of N₂O. Alternatively, a powder of a catalyst selective for the reduction of N₂O may be mixed with a binder and extruded into the honeycomb configuration. In another approach, the catalytic material may be formed in situ by preparing the honeycomb structure from a zeolitic precursor raw material which is then treated to form the zeolitic material as part of the honeycomb structure.

The following examples are presented to provide a more complete understanding of the invention. The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles and practice of the invention are exemplary and should not be construed as limiting the scope of the invention.

EXAMPLE 1 Synthesis of N₂O Reduction Catalysts

A zeolite catalyst was prepared according to the following general procedure. Zeolite Beta powder was prepared via the synthetic procedures disclosed in Example 1 (Batch 1) of commonly owned U.S. Pat. No. 4,961,917 of John W. Byrne, which is expressly incorporated by reference herein. The powder was then ion-exchanged with Fe by dispersing 212.5 g of zeolite Beta powder in a preheated, 70° C. solution consisting of 1000 g of deioinized water and 25.5 g of iron sulfate heptahydrate. The preparation was mixed for 1 hour, after which the zeolite powder was filtered from the solution followed by water washing to remove residual sulfate. The filtered cake was then mixed with deionized water in the proportion 40% zeolite/60% water by weight, and the mixture was placed in a high shear mixer to form a washcoat slurry containing zeolite Fe/Beta with a particle size 90% less than 20 μm. A monolith support of cordierite containing 100 cells per square inch of cross section was dipped into the washcoat slurry. After calcination at 400° C., the support contained 1.5 g of zeolite Fe/Beta/in³.

EXAMPLE 2 Catalytic Decomposition of N₂O

The following general procedure was utilized for catalytically converting N₂O with ammonia to gaseous nitrogen and water. A core sample of Fe/Beta on 100 CPSI honeycomb was loaded into a 1 inch reactor. A gas stream consisting of varied concentrations of N₂O and ammonia, 10% O₂, 10% H₂O, and balanced with N₂ was fed through the catalyst at a flow rate equivalent to 20,000 hour⁻¹ space velocity (“SV”), which is defined as [(gas flow rate at 25° C. (liters/hr))/(catalyst volume (liters))]. The disappearance of both N₂O and ammonia across the catalyst were measured by taking gas samples before and after passing over the catalyst. The gas samples were then measured by on-line N₂O and ammonia infrared analyzers, such as Sieman N₂O (Ultramat 5E) and NH3(Ultramat 5F) analyzers. These conversions were then measured over a temperature range of 250° C. to 450° C. The tests were conducted with varying concentrations of N₂O, and ammonia: (a) 200 ppm N₂O, 0 ppm ammonia; (b) 200 ppm N₂O, 200 ppm ammonia; and (c) 0 ppm N₂O, 200 ppm ammonia. As illustrated in FIG. 1, N₂O conversion was low (i.e., 10%) at temperatures ranging from 250° C. to 450° C. in the absence of ammonia. With 200 ppm ammoniapresent, N₂O conversion increased substantially with increasing temperature. By adding ammonia, N₂O conversion increased significantly at temperatures greater than 300° C. These data demonstrate that the presence of ammonia substantially increased the reduction rate of N₂O, even at low temperatures (i.e., above 250° C.).

FIG. 2 illustrates the disappearance of ammonia. In the absence of N₂O, ammonia conversion was very low, indicating that very little ammonia was oxidized. However, in the presence of N₂O, the ammonia disappearance rate increased substantially. Thus, it is apparent that ammonia and N₂O mutually enhance the conversion of each other to N₂ and H₂O over a zeolite-based catalyst.

EXAMPLE 3 Effect of a Zeolite Catalyst on Ammonia-mediated N₂O Reduction

In order to ascertain if ammonia-mediated N₂O reduction is unique to zeolite catalysts, the procedures described in Example 2 were conducted utilizing two other catalytic compositions, V₂O₅/TiO₂, and Pt/Au. These compositions were obtained as follows:

V₂O₅/TiO₂

130.8 g of citric acid was mixed with 1000 g of deionized water, and the mixture was heated to 80° C. to dissolve the citric acid. This solution was then combined in a mixing tank with 37.5 g of ammonium metavanadate, followed by an additional 700 g of deionized water. 1425 g of TiO₂ powder having a BET surface area of 100 m²/g was added to the solution to obtain a 2% V₂O₅/TiO₂ washcoat slurry. A monolith support of cordierite containing 100 cells per square inch of cross section was dipped into the washcoat slurry. After calcination at 400° C., the support contained 1.5 g of V₂O₅/TiO₂ catalyst powder/in³.

Pt/Au 177 g of gamma alumina powder having a BET surface area of 150 m²/g was ball milled with deionized water and acetic acid to form a 50% solid slurry. The slurry was then placed in a dispersion tank and combined with 0.83 g of Pt equivalent amine-solubilized aqueous platinum hydroxide (H₂Pt(OH)₆) solution and 0.17 g of Au equivalent aqueous (HAuCl₄·3H₂O) solution. A monolith support of cordierite containing 100 cells per square inch of cross section was dipped into the washcoat slurry. After calcination at 400° C., the support contained 1.7 g of Al₂O₃/in³, 40 g of Pt/ft³, and 8 g of Au/ft³.

FIG. 3 illustrates that for the V₂O₅/TiO₂ catalyst, there was very little N₂O reduction over the tested temperature range, with or without the presence of ammonia. Unlike the zeolite catalyst, the V₂O₅/TiO₂ catalyst did not show any activity for N₂O. FIG. 4 demonstrates that ammonia conversion is not affected by the presence of N₂O. In contrast to the zeolite catalyst, the reduction rate of N₂O over the V₂O₅/TiO₂ catalyst could not be promoted by injecting ammonia into the gas stream. These results demonstrate that the V₂O₅/TiO₂ catalyst does little to foster the interaction between N₂O and ammonia. Also, even though the V₂O₅/TiO₂ catalyst converted some ammonia at temperatures in the 350° C. to 450° C. range, the conversion was not increased when N₂O was present in the gas stream. These results show that the mutually enhanced conversion of NH₃ and N₂O to N₂ and H₂O is specific to the unique catalytic properties of the zeolite catalyst. It is clear that the combination of ammonia addition and the use of a zeolite-based catalyst is essential to enhance the N₂O removal rate.

FIGS. 5 and 6 are graphs on N₂O and ammonia conversions, respectively, for a Pt/Au catalyst. As shown in these figures, the Pt based catalyst was not active to decompose N₂O, but was very active to oxidize ammonia. When ammonia and N₂O are co-present in an oxidizing environment, the N₂O conversion could become negative, indicating that some N₂O was formed through the ammonia oxidation reaction. Thus, for this catalyst, the presence of ammonia not only fails to enhance N₂O conversion, but also negatively affects N₂O removal efficiency.

FIG. 7 and 8 are graphs of laboratory results showing the use of zeolite catalysts to achieve reduction for both NO_(x) and N₂O gases by introducing ammonia into the gas stream. For this test, the inlet gas contained 815 ppm N₂O and 52 ppm NO_(x). The conversions of N₂O and NO_(x) across a Fe/Beta catalyst were measured at 450° C. and 500° C. FIG. 7 shows that with no ammonia, there was no NO_(x) conversion, and the N₂O conversions were 30% and 78%, respectively, at 450° C. and 500° C. FIG. 8 shows that by introducing 811 ppm NH3 to the gas stream, both NO_(x) and N₂O conversions were increased substantially. For NO_(x) removal, greater than 98% conversion was achieved at both temperatures. For N₂O removal, conversion was increased to 80% at 450° C. and 99% at 500° C. These results demonstrate that the zeolite catalyst is very active to promote the selective catalytic reduction of NO_(x) with ammonia to form N₂ and water. Additionally, the combination of NH₃ and this zeolite catalyst substantially improved the N₂O conversion efficiency at the lower temperature.

FIG. 9 is a schematic of the apparatus for the control of N₂O gas. In this schematic, ammonia is introduced to a gaseous stream containing N₂O at a ratio of about 1:1. This stream is then passed through a zeolite-based catalyst, which promotes the mutually enhanced removal rates of N₂O and NH₃.

FIG. 10 is a schematic of the apparatus for the simultaneous control of N₂O and NO_(x). In this schematic, the gaseous stream is introduced with sufficient quantity of ammonia. This stream is then passed through a zeolite-based catalyst which promotes the mutually enhanced rates of N₂O and ammonia removal, as well as the selective catalytic reduction of NO_(x).

Having thus described the invention in detail, it will be recognized that such detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention, as defined by the subjoined claims. 

1. A method for ammonia-mediated N₂O and NOx reduction, comprising contacting a gas stream containing N₂O and NOx with ammonia and a catalyst comprising a BETA zeolite which is selective for the simultaneous reduction of N₂O and NOx, wherein the gas stream containing ammonia, nitrous oxide and NOx has a temperature of greater than about 250° C.
 2. A method as recited in claim 1, wherein the gas stream has a temperature of from about 350° C. to about 600° C.
 3. A method as recited in claim 1, wherein the ammonia/N2O concentration ratio is up to about 2.0 based on the total volume of the gas stream.
 4. A method as recited in claim 1, wherein the ammonia/N₂O concentration ratio is at least about 0.5 based on the total volume of the gas stream.
 5. A method as recited in claim 1, wherein the ammonia/N₂O concentration ratio is from about 0.8 to about 1.0 based on the total volume of the gas stream.
 6. A method as recited in claim 1, wherein the zeolite is ion-exchanged with at least one type of ion selected from the group consisting of Fe, Cu, Co, Ce, Pt, Rh, Pd, Ir, Mg and combinations thereof.
 7. A method as recited in claim 1 wherein the zeolite is ion-exchanged with at least one type of ion selected from the group consisting of Fe, Ce, Cu, Co and combinations thereof.
 8. A method as recited in claim 1, wherein the N₂O concentration of the gas stream is about 1% or less.
 9. A method as recited in claim 1, wherein the N₂O concentration of the gas stream is about 5,000 ppm or less.
 10. A method as recited in claim 1, wherein the N₂O concentration of the gas stream is between about 20 ppm and about 5,000 ppm. 